Field
A positive electrode composition for a lithium secondary battery and a lithium secondary battery using the same are disclosed.
Description of the Related Technology
Secondary batteries have been increasingly demanded in accordance with technology development and demand for portable devices. In particular, secondary batteries with high energy density and high voltage have been commercially available and widely used.
In general, lithium secondary batteries generate energy by intercalating and deintercalating lithium ions during the charge and discharge. The lithium secondary batteries basically include a negative electrode including a negative active material, a positive electrode including a positive active material, a separator, and an electrolyte including an organic solvent. As for the negative active material, a tin or silicon-based composite material has recently drawn attention.
The positive active material for a lithium secondary battery may include lithium-containing metal oxide such as LiCoO2, LiMn2O4, LiNiO2, LiMnO2, and the like, while the negative active material may include a lithium metal or an alloy thereof, a carbon material, and the like. The positive or negative active material for a lithium secondary battery is not basically conductive.
Accordingly, the active material particles are coated with a conductive agent the surface thereof in order to form a conductive network and increase conductivity.
The positive electrode for a lithium secondary battery is fabricated by mixing a positive active material, a binder, and a conductive agent in an organic solvent and dispersing the mixture to prepare positive electrode slurry composition, coating the positive electrode slurry composition on a positive current collector, and then, drying and compressing the coated current collector.
The binder is used to improve adherence among positive active material particles or the positive active material particles to the positive current collector. When the positive electrode slurry is dried on the positive current collector, the binder existing among the positive active material particles or between the positive active material particles and the current collector is transformed from a liquid to a solid and has adherence.
However, when polymers consisting of the binder are cross-linked in the slurry, the slurry becomes gelated. The gelation illustrates a phenomenon that slurry is transformed from sol to gel. This gelation makes to difficult to uniformly coat the slurry on the current collector and, if ever, decreases adherence among the particles or of the particles to the current collector. The deteriorated adherence among the particles may cause detachment of the particles from the surface of the positive electrode and resultantly deteriorate battery safety. In other words, the particles detached from the positive electrode due to the insufficient adherence may cause a microshort inside a battery and deteriorate battery performance.
Furthermore, some large microshorts may cause a short circuit and set a fire. In addition, when the adherence of the particles to the current collector is deteriorated, electrons may have resistance during the movement from the particles to the current collector, which may decrease an electrical conductivity rate and resultantly, deteriorate high rate capability and cycle-life characteristics. Furthermore, when the particles coated to be hundreds of micrometer(μm)-thick on the current collector are pressed after coating the slurry, the particles lack of adherence and stick to a constantly-rolling roll and may cause a surface defect or an electrode defect due to overpressure, decreasing a manufacturing process yield.